1. Field of the Invention
The invention relates to high temperature superconducting composite materials, and more particularly to a high temperature superconducting composite material made by filling the open cells of a silver, silver alloy, gold or gold alloy foam with a high temperature superconducting ceramic oxide.
2. Description of the Related Art
Superconducting ceramic oxides with critical temperatures that exceed the temperature of liquid nitrogen have been developed in recent years. However, the full potential of these materials for transporting power or generating magnetic fields has not been realized because the ceramic oxides are hard, brittle and difficult to form in proper geometries (wires, rods or tapes) with the requisite composition and structural orientation.
Superconducting rods, wires or tapes may be fabricated by packing the ceramic oxide powder into a metal tube and then drawing the tube into a wire or rod or flattening it into a tape. Methods of making single strand conductors are disclosed in U.S. Pat. No. 4,980,964 to Boeke and U.S. Pat. No. 5,206,211 to Meyer. To create a multifilamentary conductor for use in magnets the wire can be rebundled and then redrawn or rolled into a tape. A method of fabricating superconducting ceramic tape is to coat the ceramic onto one or both sides of a metal tape. Various other geometric configurations for superconducting wire with multiple filaments are disclosed in U.S. Pat. No. 4,929,596 to Meyer et al, U.S. Pat. No. 4,849,288 to Schmaderer et al, U.S. Pat. No. 5,374,612 to Ito et al. U.S. Pat. No. 5,017,553 to Whitlow et al and U.S. Pat. No. 5,347,085 to Kikuchi et al. Conductors as described above are limited in their current carrying capacity because individual superconducting filaments are subject to material inhomogeneities.
Other approaches to reinforcing superconducting ceramics, making them stronger and easier to fabricate, are to disperse metal particles throughout a superconducting matrix or to disperse superconducting ceramic particles throughout a metal matrix. U.S. Pat. No. 5,082,826 to Ferrando discloses a superconducting ceramic powder in which powder particles are coated with silver. U.S. Pat. No. 4,983,574 to Meyer describes a conductor having particles of ceramic sintered high-temperature superconductor embedded in a metal matrix. U.S. Pat. No. 5,306,704 to Gleixner et al discloses a method of dispersing a metal homogeneously throughout a superconducting material by blending the metal and a superconducting oxide, melting the blend and blowing the blend to form glasseous filaments. Superconducting materials as described above contain either discontinuous ceramic oxide particles or discontinuous metal particles. The dispersion of metal throughout a ceramic matrix or the dispersion of ceramic throughout a metal matrix provides improved strength and flexibility, but results in poorer superconducting properties and thermal stability. U.S. Pat. No. 5,470,821 to Bong et al describes a bulk composite superconducting material made by combining ceramic oxide and metallic particulates. The composite is compressed so that the metallic material fills the interstices between the ceramic oxide particles.
It is an object of the invention to provide a high temperature superconducting ceramic oxide composite with a high current carrying capacity.
It is a further object of the invention to provide a high temperature superconducting ceramic oxide composite that is mechanically strong.
It is a further object of the invention to provide a high temperature superconducting ceramic oxide composite that has a large interface area with silver, silver alloy, gold or gold alloy uniformly spaced throughout the composite.
It is a further object of the invention to provide a high temperature superconducting ceramic oxide composite that allows for rapid and uniform oxygenation of the superconducting ceramic oxide.
It is a further object of the invention to provide a high temperature superconducting ceramic oxide composite that provides high thermal conductivity to minimize the development of hot spots during operation as a superconductor.
It is a further object of the invention to provide a high temperature superconducting ceramic oxide composite that is easy and inexpensive to fabricate.
These and other objects are accomplished by a method of making a high temperature superconducting composite comprising the steps of providing a reticulated foam structure made of a metal selected from the group consisting of silver, silver alloy, gold and gold alloy, the reticulated foam structure having continuous ligaments defining a plurality of continuous open cells, filling the continuous open cells of the reticulated foam structure with a high temperature superconducting ceramic oxide or precursor, compacting the filled structure, and heating the compacted structure to melt and/or texture the high temperature superconducting ceramic oxide or precursor to form a continuous region of high temperature superconducting ceramic oxide throughout the compacted reticulated foam structure. Making a composite by the method outlined above results in a composite that is mechanically strong, and has a large interface area between the superconducting ceramic oxide and the metal, which allows a uniform current flow and a high current density. Since oxygen diffuses more rapidly through the metals listed above than it does through a ceramic oxide, the large interface area allows for more rapid and more homogeneous oxygenation. Because the metals are more thermally conductive than ceramic oxide, the thermal stability of the superconducting ceramic oxide is enhanced.